fwd/core/default.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::default_` and `boost::hana::is_default`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_CORE_DEFAULT_HPP
0011 #define BOOST_HANA_FWD_CORE_DEFAULT_HPP
0012
0013 #include <boost/hana/config.hpp>
0014
0015
0016 namespace boost { namespace hana {
0017     //! @ingroup group-core
0018     //! Mark a tag-dispatched method implementation as a default implementation.
0019     //!
0020     //! When defining a new concept with tag-dispatched methods, it is
0021     //! sometimes possible to provide a default implementation for some
0022     //! method(s). Making `default_` a base class of such a default
0023     //! implementation makes it possible to detect whether the method
0024     //! was dispatched to the default implementation afterwards.
0025     //!
0026     //!
0027     //! Example
0028     //! -------
0029     //! @include example/core/default.cpp
0030     struct default_ { };
0031
0032     //! @ingroup group-core
0033     //! Returns whether a tag-dispatched method implementation is a default
0034     //! implementation.
0035     //!
0036     //! Given a tag-dispatched method implementation `method_impl<T...>`,
0037     //! `is_default<method_impl<T...>>` returns whether `method_impl<T...>`
0038     //! is a default implementation. Note that if there is no default
0039     //! implementation for the method, then `is_default` should not be
0040     //! used unless a static assertion saying that "the method is not
0041     //! implemented" is acceptable.
0042     //!
0043     //!
0044     //! Example
0045     //! -------
0046     //! @include example/core/default.cpp
0047 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0048     template <typename Method>
0049     struct is_default { see documentation };
0050 #else
0051     template <typename T, typename = void>
0052     struct is_default;
0053 #endif
0054 }} // end namespace boost::hana
0055
0056 #endif // !BOOST_HANA_FWD_CORE_DEFAULT_HPP